1. Field of Invention
The present invention relates to a method and apparatus for optically measuring crosswinds. More specifically, the present invention relates to a method and apparatus for optically measuring crosswinds by a pattern recognition technique.
2. Description of Prior Art
Techniques have been developed for measuring average crosspath winds by observing the characteristics of an optical beam in a receiving plane. The basic measurement involves a laser source, and correlation measurements within the diffraction pattern produced at the receiver, by the atmospheric turbulence interacting with the transmitted beam. The turbulence is considered to be frozen and the bulk air motion carries the turbulence through the beam. This produces a diffraction pattern velocity which is related to the crosswind speed.
Lawrence, Ochs and Clifford have developed a system for obtaining wind speed from a measurement of the time change in the correlation function observed by two closely spaced detectors. This system is described in a publication entitled "Use of Scintillations to Measure Average Wind Across a Light Beam", Applied Optics, 11, February 1972, p. 239. The system produces good results when operating in weak turbulence or over short paths in strong turbulence. However, the system performs poorly with strong turbulence over long paths. The effects of strong turbulence on the observed correlation characteristics are not well understood, so system parameters cannot be selected to obtain an accurate wind measurement. This is most probably due to the fact that the temporal spectral density of the amplitude fluctuations of a laser beam through strong turbulence does not follow the inverse root path length, L.sup..sup.-1/2, prediction.
Optical devices currently used for crosswind measurements are calibrated and results interpreted on the basis of theoretical predictions. However, under strong turbulence conditions, the experimental observations do not compare well with the theoretical predictions. Thus, the optical device performance deteriorates with increased turbulent intensity.